Global mobile communications systems, such as the proposed Odyssey.TM. system described in patent U.S. Pat. No. 5,433,726, require both mobile link antennas and feeder link antennas for the spacecraft. The mobile link antennas establish the communication link with mobile users and the feeder link antennas relay those communications to earth stations where they are connected to the world-wide public switched telephone network.
The plurality of the antennas carried by the spacecraft warrants stringent weight control for the antennas. The large number of antennas required in the system overall, as example a total of forty five antennas in Odyssey, the multiple of three antennas per spacecraft and fifteen spacecrafts, also requires low unit production cost. Such cost and weight constraints are in addition to the antenna's demonstrating acceptable levels of RF performance. The link antenna may be required to simultaneously or alternately transmit and receive circularly polarized (CP) signals at two different frequencies, one frequency typically being separated from the other by at least ten per cent of that one frequency. It should also do so with high gain and low sidelobe levels. Such link antenna may also be used to transmit two separate frequencies or to receive two separate frequencies. Advantageously, the antenna described herein achieves these requirements.
In prior satellite communications systems, the architecture has been to attach the respective antenna, comprising a parabolically curved reflector and feed horn, to the electronics box or container that houses the electronics. The entire assembly, antenna and electronics container, is then placed on gimbals that position or steer the antenna to the earth station.
Two factors contribute to the heavy weight of such a system. First, to maneuver a large mass and therefore the momentum, a heavy duty gimbal system is necessary. As example, the electronics box alone weighs more than 18 Kg each. Second, to secure the heavy electronics and antenna assembly in place during the launching vibration, requires the use of a heavy caging structure.
An object of the present invention, therefore, is to reduce the weight of a feeder link antenna system, particularly, to reduce the weight of the ancillary equipment required for antenna system transport in space craft, such as the caging structure.
Further the feed horn assembly used in those prior systems is relatively simple in appearance. It includes a horn and a waveguide transmission line containing, in serial order, a rexolite rod, a transmit polarizer, a transmit orthomode transducer, a receive polarizer and a receive mode launcher. In practice it is found that such known structure required adherence to strict manufacturing tolerances. Following manufacture, the prior feed system required adjustment and labor intensive tuning to ensure its proper electrical performance. Since any adjustment to one component in that feed system influenced the electronic characteristices of the other components in that feed by means of electromagnetic interaction, tuning often involved iterative cycles of tuning, testing, and inspection.
Another object of the invention, thus, is to provide a dual band feed horn assembly that has less restrictive dimensional tolerance requirements than heretofore, avoids time consuming and laborious testing and adjustment procedures, is less costly to manufacture, and is less temperature sensitive than prior dual mode feed horn assemblies.